UNSEEDED American Todd Martin, in the biggest victory of his career and the biggest surprise so far at Wimbledon, ousted last year's runner-up, Goran Ivanisevic, in a third-round upset yesterday. Fifth seed Ivanisevic fell apart at the end of his third consecutive five-set match, double-faulting on the second match point to give Martin a startling 2-6, 7-6 (7-3), 6-7 (4-7), 7-5, 6-0 victory. It was the 15th double fault for the Croatian, who was twice warned for outbursts of temper. He left the grounds quickly, risking a fine by spurning an obligatory post-match news conference. Both number two seeds reached the fourth round, Martina Navratilova with a 6-1, 6-0 rout of Canada's former Hongkong Federation Cup player Patricia Hy, and Stefan Edberg with a sloppy victory, losing his serve six times before beating British long-shot Chris Wilkinson 6-4, 7-5, 6-3. Women's fifth seed Mary Joe Fernandez suffered her worst defeat in three years, 6-0, 6-1 at the hands of doubles partner Zina Garrison-Jackson, the losing Wimbledon finalist in 1990. The men's 12th seed, Michael Chang, fell short in an attempted five-set comeback, but it was scarcely an upset. Unseeded American David Wheaton, who won 6-4, 6-4, 5-7, 4-6, 6-4 in four hours, 22 minutes, was a semi-finalist in 1991 and has lost only once to Chang in seven meetings. Martin, 22, a second-round loser last year, his only other Wimbledon appearance, looked cooler on Court One than Ivanisevic. ''I tried very hard to keep my emotions to myself. I did a pretty good job,'' he said. ''You don't really think of patience on grass, but it had a lot to do with it today, because Goran really dominated me in the first set.'' Navratilova has dropped only eight games in her first three matches, while top-seeded Steffi Graf, seeking her fifth title, has lost only three and twice shut out opponents. Graf and Navratilova are odds-on favourites to meet in the final on July 3. ''Every match I've been playing a little better,'' said Navratilova, 36, who is seeking her 10th Wimbledon singles title. In other women's third-round matches yesterday, eighth seed Jana Novotna beat American Marianne Werdel, 6-3, 6-1 and ninth-seeded Anke Huber beat Florencia Labat of Argentina, 6-4, 6-3. Nathalie Tauziat of France, the 16th seed, put out American Lindsay Davenport, 6-3, 6-7 (7-5). Unseeded Natalia Zvereva of Belarus beat Sabine Appelmans of Belgium 6-3, 6-4; and fellow non-seed Miriam Oremans of the Netherlands beat Italy's Gloria Pizzichini, 6-1, 6-4. Among the men, the 13th seed, Wayne Ferreira of South Africa, advanced past Henrik Holm of Sweden, 6-4, 6-3, 7-6 (7-2), and Wally Masur of Australia won 6-3, 6-4, 6-7 (5-7), 6-7 (7-3), 6-4 against Ivan Lendl's second round conqueror Arnaud Boetsch of France. Unseeded American Richard Matuszewski, in his best-ever Grand Slam, beat Javier Frana of Argentina, 6-4, 3-6, 6-2, 6-2, to earn a fourth-round match with Edberg. Wilkinson, ranked 143rd in the world, excited the partisan Centre Court crowd during a bizarre second set which featured seven service breaks, three of them for him. The most astonishing exchange came with Edberg serving at 5-3. Behind 0-15, Edberg slammed an overhead into the corner, Wilkinson dived head-first and somehow lobbed it back over the net. Edberg prepared for another overhead while Wilkinson lay on the grass, but smashed the ball into the middle of the net. Edberg hit an easy volley into the net on the next point, lost the game, and Wilkinson held serve at love to even the set at 5-5 before Edberg prevailed. ''It was a very unusual match,'' Edberg said. ''I didn't play very well. Some of the shots I missed were just out of this world.'' With a rest day today, Graf's fourth-round opponent tomorrow will be Meredith McGrath, a qualifier ranked 148th. Top men's seed Pete Sampras faces 332nd-ranked Andrew Foster of Britain.